Back in 2013 I created http://slotted.co. It's a general purpose signup sheet app. Common use cases:

- college professor office hours

- open enrollment consultations

- comedy open mics

- auditions & interviews

- startup accelerator mentor meetings

- any kind of one-on-one meetings

- potluck parties

- volunteer signups

- pizza orders

I started monetizing in 2015 and now earn 5-figures averaging < 5 hours or effort per month. Now and then I do a batch of work to add a new feature.